Scala3 should support Unicode supplementary characters

[magnolia-k]

scala/bug#1406

scala/scala#9687

[som-snytt]

I will forward port as soon as dust settles. Sorry for breaking previous rule to contribute here and then backport. My excuse is Lukas gives quick feedback and fixes your mistakes, he is Copilot for Scala corpus.

[odersky]

@som-snytt Thanks!